Solar selective coatings are used as absorbers for harnessing solar energy for various applications. One of the essential requirements of solar selective absorbers is their stable structural composition when they operate at high temperatures. Optical properties of these coatings should not degrade with respect to rise in temperature or over a period of use. The main utility of the present invention is for high temperature applications particularly in solar steam generators and steam turbines for producing the electricity. It also finds applications for obtaining decorative coatings, wherein the coating should have higher thermal stability, such as exhaust and silencer pipes in automobile engines.
In recent years a greater attention has been shown in harnessing alternative sources of energy like solar energy for industrial applications, where the electricity is used in large quantities to produce steam for various industrial processes. Generally, concentrating type solar collectors are popularly used in industries for high temperature applications. Solar selective coatings applied to solar absorbers have been proved as an efficient method for harnessing the solar energy on large scale. These solar selective coatings are deposited by both wet and dry methods on the substrates. Most of the coatings obtained from the wet methods do not exhibit higher thermal and chemical stability. In addition, these processes are not environmentally friendly.
Conventionally, black chrome is used as an excellent solar selective coating for hot-water applications, which is deposited by electroplating an aqueous solution of sulphate-free chromic acid containing certain additives. This is a wet process and utilizes hexavalent chromium. Hexavalent chromium ions are known carcinogens and are being phased out in many applications. When chromic acid solution is electrolyzed it produces a lot of hydrogen and oxygen, which carry chromic acid and escapes to atmosphere, affecting the environment, thus causing pollution problems. Any chromic acid containing solution cannot be discharged to the drainage because it causes soil and ground water pollution problems. In general, black chrome coatings are used for low temperature applications like solar water heating. In evacuated tubes, black chrome coatings have also been used for steam generation at low pressures, wherein the application temperature is around 200° C. However, for high-temperature applications, like solar steam turbines, black chrome coatings are not recommended, since they undergo compositional changes.
Among the dry coating methods, chemical vapor deposition (CVD) generally utilizes higher deposition temperature and also toxic chemicals. The solar selective multilayer coatings of the present invention have been deposited by using a planar reactive DC magnetron sputtering process. The planar reactive DC magnetron sputtering process is most widely used physical vapor deposition (PVD) method. Also, DC magnetron sputtering is a dry, clean and eco-friendly green process for depositing a variety of coatings. In harnessing solar energy for steam generation, coatings with higher thermal stability are required. In the sputtering method it is possible to precisely control the stoichiometry of the coating and also the deposition temperature is generally low (room temperature to 400° C.). High melting point metal nitrides (including transition metal nitrides) with controlled microstructure can be deposited by sputtering, which are chemically inert and very hard. Furthermore, this method can be scaled up for industrial applications.
Prior-art search was made in public domain for patent as well as non-patent literature to differentiate the present invention with other inventors' work. Some of the works, which are related to the field of the present invention, are discussed below.
Reference may be made to “Pt—Al2O3 selective cermet coatings on superalloy substrates for photo-thermal conversion up to 600° C.” by T. K. Vien et al. [Thin Solid Films 126 (1985) 17], wherein platinum-aluminum oxide (Pt—Al2O3) cermet solar selective films have been deposited using a radio frequency (RF) sputtering technique. They obtained coatings with α=0.92 and ε300° C.=0.14 on stainless steel and superalloy substrates. They claimed that these coatings were stable up to 600° C. on superalloy substrates after annealing in hydrogen atmosphere. In many applications the solar selective coatings are required to have higher thermal stability in air. This work has discussed thermal stability only in hydrogen atmosphere but not in air or vacuum.
Reference may be made to “Recent progress in high-temperature solar selective coatings” by Q.-C. Zhang et al. [Solar Energy Materials & Solar Cells 62 (2000) 63 and references therein], wherein a series of metal-AlN and Mo—Al2O3 cermet materials have been deposited by a DC magnetron sputtering process. They have reported a solar absorptance of 0.96 and a hemispherical emittance of 0.11 at 350° C. for Al2O3/Mo—Al2O3 (low metal volume fraction)/Mo—Al2O3 (high metal volume fraction)/Mo films. Solar absorptance of 0.96 and near normal emittance of 0.08 at 350° C. were achieved for Mo—Al2O3 cermet coatings on copper infrared reflectors. They have also reported that stainless steel-aluminum nitride (SS-AlN), tungsten-aluminum nitride (W—AlN) and molybdenum-aluminum nitride (Mo—AlN) cermet coatings exhibit good thermal stability in the temperature range of 350-500° C. in vacuum. They have not reported the thermal stability of these coatings in air.
Reference may be made to “Sputter etched metal solar selective absorbing surfaces for high temperature thermal collectors” by G. L. Harding and M. R. Lake [Solar Energy Materials 5 (1981) 445], wherein sputter etched Cu-SS-Ni substrates have been produced in a cylindrical magnetron with α=0.92 and ε27° C.=0.12-0.25. Sputter etched copper surfaces were stable in vacuum up to 400° C. and stainless steel surfaces were stable in vacuum up to 500° C. These surfaces deteriorated in air at 400° C.
Reference may be made to “High-temperature optical properties and stability of AlxOy—AlNx—Al solar selective absorbing surface prepared by DC magnetron reactive sputtering” by S. Yue et al. [Solar Energy Materials & Solar Cells 77 (2003) 393], wherein AlxOy—AlNx—Al solar selective absorber coatings have been deposited using DC magnetron reactive sputtering of aluminum alloy in air and argon. These coatings were found to be stable up to 600° C. for 30 minutes in 4.5×10−3 Pa vacuum with α=0.94 and ε=0.07. After heating at 450° C. for 10 hours in vacuum the specimen showed α=0.93 and ε=0.07. Tests in air have not been conducted.
Reference may be made to Chinese Patent No. 01138135.3, wherein solar selective films consisting of reflection and absorption layers were deposited on metal and glass substrates using magnetic control reactive vacuum deposition system. The absorption layer was deposited by sputtering Ti and Al as cathodes in N2, air medium and N2+O2 to form (AlN+TiN)—AlTi films. The infrared reflection layer was Ti+Al of thickness 0.09 μm (900 Å). The absorber layer was (AlN+TiN) and AlNO+TiNO of thickness 0.15 to 0.4 μm. The thickness of the reflection reduction layer was 0.04-0.2 μm. The film was heated for 250 hours at 350° C. or 50 hours at 400° C. or 80 hours at 450° C. in air. They have claimed α=0.93 and ε=0.06-0.10. The Ti+Al and TiN coatings used in this invention are susceptible to oxidation at higher temperature, thus affecting the optical properties of the solar selective coatings.
Reference may also be made to “Optimization of SiO2—TiNxOy—Cu interference absorbers: numerical and experimental results” by M. P. Lazarov et al. [Proceedings of the Society for Photothermal Instrumentation Engineers (SPIE) 2017 (1993) 345], wherein TiNxOy solar selective coatings have been developed on Al and Cu substrates using activated reactive evaporation with SiO2 as antireflection coating. Best coatings showed α=0.94 and ε100° C.=0.04. They claimed that the coatings withstand breakdown in cooling fluid and vacuum if mounted on evacuated collector.
Reference may also be made to TiNOX based absorber coatings [http://www.tinox.com/], wherein TiNOX GmbH, is currently marketing TiNOX solar absorber coatings. They use SiO2 as the protective layer. TiNOX has reported α=0.94 and ε=0.05 for their coatings. It has been claimed that non-vacuum collectors equipped with TiNOX reach temperature up to 220° C. and vacuum tubes reach temperature as high as 325° C.
Reference may also be made to “TiAlON black decorative coatings deposited by magnetron sputtering” by R. Luthier and F. Levy [Vacuum 41 (1990) 2205], wherein titanium-aluminum oxynitride (TiAlON) black decorative coatings have been developed using planar RF magnetron sputtering of Al2O3+1.5TiN target on sapphire substrates. They have claimed that the coatings were stable up to 900° C. under vacuum. However, they have not studied the optical properties of the coatings.
Reference may also be made to “Performance of oxygen-rich TiAlON coatings in dry cutting application” by K. Tonshoff et al. [Surface and Coatings Technology 108-109 (1998) 535], wherein graded and multilayer coatings of TiAlN/TiAlON have been deposited with varying oxygen contents for dry cutting applications using an RF assisted magnetron sputter ion plating and a conventional magnetron sputter ion plating. They have studied only wear resistance in dry drilling of tempered steel for 7-layered film with alternating films of TiAlN and TiAlON. They have not discussed anything about solar selective properties of these coatings.
As seen from the prior art literature, various researchers have tried to manufacture solar selective coatings with higher thermal stability. Best coatings as reflected in the prior art have thermal stability up to 400-450° C. and solar selectivity in the range of 9-10. But still the problem of improved thermal stability above 450° C. exists. It is also desired that the solar selective coatings should have high oxidation resistance, chemical inertness and high hardness for high temperature applications. There is a definite scope to provide solution to such kind of problems in order to expand the life of the solar selective coating having higher thermal stability. Hence, there is a definite need to manufacture such solar selective coatings having higher thermal stability, which obviate the drawbacks of the prior art, as mentioned earlier.